This application claims the benefit of a Japanese Patent Application No. 2000-373375 filed Dec. 7, 2000, in the Japanese Patent Office, the disclosure of which is hereby incorporated by reference.
1. Field of the Invention
The present invention generally relates to magnetic recording media and magnetic storage apparatuses, and more particularly to a magnetic recording medium and a magnetic storage apparatus which are suited for high-density recording.
Due to developments made in the information processing technology, there are increased demands to further increase the recording density of magnetic recording media. Of the characteristics which are required by the magnetic recording medium to satisfy such demands, the medium noise needs to be reduced and the thermal stability needs to be improved in the case of a hard disk.
2. Description of the Related Art
The recording density of longitudinal magnetic recording media, such as magnetic disks, has been increased considerably, due to the reduction of medium noise and the development of magnetoresistive and high-sensitivity spin-valve heads. A typical magnetic recording medium is comprised of a substrate, an underlayer, a magnetic layer, and a protection layer which are successively stacked in this order. The underlayer is made of Cr or a Cr-based alloy, and the magnetic layer is made of a Co-based alloy.
Various methods have been proposed to reduce the medium noise. For example, Okamoto et al., xe2x80x9cRigid Disk Medium For 5 Gbit/in2 Recordingxe2x80x9d, AB-3, Intermag ""96 Digest proposes decreasing the grain size and size distribution of the magnetic layer by reducing the magnetic layer thickness by the proper use of an underlayer made of CrMo, and a U.S. Pat. No. 5,693,426 proposes the use of an underlayer made of NiAl. Further, Hosoe et al., xe2x80x9cExperimental Study of Thermal Decay in High-Density Magnetic Recording Mediaxe2x80x9d, IEEE Trans. Magn. Vol.33, 1528 (1997), for example, proposes the use of an underlayer made of CrTiB. The underlayers described above also promote c-axis orientation of the magnetic layer in a plane which increases the remanence magnetization and the thermal stability of written bits. In addition, proposals have been made to reduce the thickness of the magnetic layer, to increase the resolution or to decrease the transition width between written bits. Furthermore, proposals have been made to decrease the exchange coupling between grains by promoting more Cr segregation in the magnetic layer which is made of the CoCr-based alloy.
However, as the grains of the magnetic layer become smaller and more magnetically isolated from each other, the written bits become unstable due to thermal activation and to demagnetizing fields which increase with linear density. Lu et al., xe2x80x9cThermal Instability at 10 Gbit/in2 Magnetic Recordingxe2x80x9d, IEEE Trans. Magn. Vol.30, 4230 (1994) demonstrated, by micromagnetic simulation, that exchange-decoupled grains having a diameter of 10 nm and ratio KuV/kBTxcx9c60 in 400 kfci di-bits are susceptible to significant thermal decay, where Ku denotes the magnetic anisotropy constant, V denotes the average magnetic grain volume, kB denotes the Boltzmann constant, and T denotes the temperature. The ratio KuV/kBT is also referred to as a thermal stability factor.
It has been reported in Abarra et al., xe2x80x9cThermal Stability of Narrow Track Bits in a 5 Gbit/in2 Mediumxe2x80x9d, IEEE Trans. Magn. Vol.33, 2995 (1997) that the presence of intergranular exchange interaction stabilizes written bits, by MFM studies of annealed 200 kfci bits on a 5 Gbit/in2 CoCrPtTa/CrMo medium. However, more grain decoupling is essential for recording densities of 20 Gbit/in2 or greater.
The obvious solution has been to increase the magnetic anisotropy of the magnetic layer. But unfortunately, the increased magnetic anisotropy places a great demand on the head write field which degrades the xe2x80x9coverwritexe2x80x9d performance which is the ability to write over previously written data.
In addition, the coercivity of thermally unstable magnetic recording medium increases rapidly with decreasing switching time, as reported in He et al., xe2x80x9cHigh Speed Switching in Magnetic Recording Mediaxe2x80x9d, J. Magn. Magn. Mater. Vol.155, 6 (1996), for magnetic tape media, and in J. H. Richter, xe2x80x9cDynamic Coervicity Effects in Thin Film Mediaxe2x80x9d, IEEE Trans. Magn. Vol.34, 1540 (1997), for magnetic disk media. Consequently, the adverse effects are introduced in the data rate, that is, how fast data can be written on the magnetic layer and the amount of head field required to reverse the magnetic grains.
On the other hand, another proposed method of improving the thermal stability increases the orientation ratio of the magnetic layer, by appropriately texturing the substrate under the magnetic layer. For example, Akimoto et al., xe2x80x9cRelationship Between Magnetic Circumferential Orientation and Magnetic Thermal Stabilityxe2x80x9d, J. Magn. Magn. Mater. vol.193, pp.240-242 (1999), report through micromagnetic simulation, that the effective ratio KuV/kBT is enhanced by a slight increase in the orientation ratio. This further results in a weaker time dependence for the coercivity which improves the overwrite performance of the magnetic recording medium, as reported in Abarra et al., xe2x80x9cThe Effect of Orientation Ratio on the Dynamic Coercivity of Media for  greater than 15 Gbit/in2 Recordingxe2x80x9d, IEEE Trans. Magn. vol.35, pp.2709-2711, 1999.
Furthermore, keepered magnetic recording media have been proposed for thermal stability improvement. The keeper layer is made up of a magnetically soft layer parallel to the magnetic layer. This soft layer can be disposed above or below the magnetic layer. Oftentimes, a Cr isolation layer is interposed between the soft layer and the magnetic layer. The soft layer reduces the demagnetizing fields in written bits on the magnetic layer. However, coupling the magnetic layer to a continuously-exchanged coupled soft layer defeats the purpose of decoupling the grains of the magnetic layer. As a result, the medium noise increases.
In order to overcome the problems described above, a magnetic recording medium was proposed in a U.S. patent application Ser. No. 09/425,788 filed Oct. 22, 1999, the disclosure of which is hereby incorporated by reference. This proposed magnetic recording medium includes at least one exchange layer structure, and a magnetic layer formed on the exchange layer structure, where the exchange layer structure includes a ferromagnetic layer and a nonmagnetic coupling layer provided on the ferromagnetic layer and under the magnetic layer, so that magnetization directions of the magnetic layer and the ferromagnetic layer are antiparallel. This proposed magnetic recording medium can improve the thermal stability of written bits and reduce the medium noise, so as to enable a reliable high-density recording without degrading the overwrite performance.
In addition, when two magnetic layers have mutually different thicknesses and have antiparallel magnetization directions in this proposed magnetic recording medium, portions of the magnetizations of the two magnetic layers cancel each other. As a result, an effective grain size of the magnetic layers can be increased substantially without affecting the resolution. Accordingly, it is possible to realize a magnetic recording medium having improved thermal stability.
In other words, this proposed magnetic recording medium employs a basic structure for improving the thermal stability of written bits, and for reducing the medium noise. When an external recording magnetic field is applied to this proposed magnetic recording medium, the magnetization directions of the magnetic layer and the ferromagnetic layer become parallel or closer to parallel, and the magnetization direction of the ferromagnetic layer is inverted when the recording magnetic field is thereafter reduced to zero (residual magnetization state) and becomes antiparallel to the magnetization direction of the magnetic layer. Therefore, it is important to invert the magnetization direction of the ferromagnetic layer to increase the total thickness of the magnetic layers and to realize a magnetic recording medium having an improved thermal stability.
In order to further improve the functions of this proposed magnetic recording medium, it is desirable that the exchange-coupling field for making the magnetization directions of the magnetic layer and the ferromagnetic layer antiparallel is generated with a sufficient intensity.
On the other hand, when mass production of the magnetic recording media is taken into consideration, it is desirable to quickly and positively judge, in a simple manner, that the exchange coupling is positively realized. However, no method has been proposed which would enable such a quick and positive judgement to be made in a simple manner with respect to the exchange coupling which is realized.
Accordingly, it is a general object of the present invention to provide a novel and useful magnetic recording medium and magnetic storage apparatus, in which the problems described above are eliminated and the desired exchange-coupling field is generated.
Another and more specific object of the present invention is to provide a magnetic recording medium and a magnetic storage apparatus, which can further improve the thermal stability of written bits and realize a reliable high-density recording.
Another and more specific object of the present invention is to provide a magnetic recording medium and magnetic storage apparatus, which can quickly and positively judge, in a simple manner, whether or not an exchange coupling is positively realized.
Still another object of the present invention is to provide a magnetic recording medium comprising at least one magnetic layer, a ferromagnetic layer, a nonmagnetic coupling layer provided between the magnetic layer and the ferromagnetic layer, and a coupling intensifying region which intensifies an exchange coupling strength between the magnetic layer and the ferromagnetic layer, where the coupling intensifying region is provided at, at least one of a boundary between the nonmagnetic layer and the magnetic layers. According to the magnetic recording medium of the present invention, it is possible to intensify the exchange coupling field within the magnetic recording medium and further improve the thermal stability. In addition, it is possible to quickly and positively judge, in a simple manner, whether or not an exchange coupling is positively realized, thereby making the magnetic recording medium suited for mass production.
A further object of the present invention is to provide a magnetic storage apparatus comprising at least one magnetic recording medium including at least one magnetic layer, a ferromagnetic layer, a nonmagnetic coupling layer provided between the magnetic layer and the ferromagnetic layer, and a coupling intensifying region which intensifies an exchange coupling strength between the magnetic layer and the ferromagnetic layer, where the coupling intensifying region is provided at, at least one of a boundary between the nonmagnetic layer and the magnetic layers. According to the magnetic storage apparatus of the present invention, it is possible to improve the thermal stability of written bits, reduce the medium noise, and carry out a high-density recording having a high reliability without adversely affecting the performance of the magnetic recording medium. In addition, it is possible to quickly and positively judge, in a simple manner, whether or not an exchange coupling is positively realized, thereby making the magnetic recording medium and thus the magnetic storage apparatus suited for mass production.
Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings.